1. Field of Invention
This invention relates generally to the removal of vapor phase contaminants from a gas stream. More particularly, this invention relates to the removal of trace amounts of vapor phase air toxics, such as mercury, from the flue gas of a combustion process.
2. Description of the Related Art
The 1990 Clean Air Act Amendments, Title III, require major sources of air emissions to limit the discharge of certain chemical species. Certain of these chemical species are categorized as air toxics, and major sources are required to limit emissions to 10 tons per year for any given air toxin. Certain of these species may be present in the flue gas emitted from combustion processes, and therefore, cost-effective methods for controlling emissions of these species are of significant interest to the operators of these processes.
Air toxics and other species regulated by the 1990 Clean Air Act Amendments can be distributed in both the vapor phase and the solid phase in the flue gas from a combustion process. Typically the air toxics are concentrated in the solid phase or particulate matter and can be effectively removed by the use of a particulate collection device such as an electrostatic precipitator or fabric filter. Air toxics, such as mercury, that are present in the vapor phase are typically found in very low concentrations, for example, parts per million or less, making removal difficult.
Several approaches are currently being evaluated to remove mercury from gas streams. These techniques involve injecting sorbents into the gas stream before a particulate collection device, passing the gas stream through a fixed or fluidized sorbent bed or structure, or using a wet scrubbing system.
Approaches using fixed bed technologies normally pass the mercury containing gas through a bed consisting of sorbent particles or various structures such as honeycombs, screens and fibers that are coated with sorbents. Common sorbents include activated carbon and noble metal such as gold and silver. In many cases where noble metals are used, only the surface layer of the structure is coated with the noble metal sorbent while the structure itself is made of ceramic or metallic materials. The sorbents on these fixed structures can be periodically regenerated by heating the structure and driving off the adsorbed mercury. The mercury driven off can be recovered or removed separately.
There are, however, several disadvantages of such fixed bed systems. Gas streams such as those from power plant coal combustion contain significant fly ash which can plug the bed structures and thus the beds need to be removed frequently from operation for cleaning. Alternatively, these beds must be located downstream of a separate particulate collector. The beds must be taken off line periodically for regeneration, thereby necessitating a second bed to remain on-line while the first one is regenerating. These beds also require significant space and are very difficult to retrofit into existing systems. For example, retrofitting such a system into the existing ductwork of a power plant could require major structural modifications.
In view of the foregoing, there exists a need for an improved method for removing vapor phase contaminants, such as mercury, from a gas stream.